The present invention relates in general to power supply regulation and, more particularly, to a switching regulator capable of receiving feedback information of opposite phase.
Most if not all electronic devices require a DC voltage of appropriate level for proper operation. The DC voltage is typically derived from an AC power source, e.g. by plugging a power supply into a wall socket. The AC voltage available at the wall socket is converted to a DC bulk voltage by a full-wave rectifier diode bridge. The DC bulk voltage is further converted to a regulated DC output voltage by a switching power supply.
The switching power supply uses a transformer, or an inductor depending on the configuration, as an energy transfer element. For example, a flyback-type power supply has a power switching transistor coupled to one side of the primary winding of a transformer. The power transistor turns on and off as determined by a regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops an output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer. The voltage across the capacitor provides the DC output voltage of the switching power supply.
The DC output voltage increases and decreases with the applied load. An increasing load decreases the DC output voltage and a decreasing load increases the DC output voltage. The DC output voltage, or a representation thereof, is fed back to the regulator circuit to allow the switching power supply to compensate for load variation. As the load increases, the DC output voltage decreases which causes the regulator to leave the power transistor on for a longer period of time to store more energy in the magnetic field. The additional energy is transferred to the secondary winding during the off time of the power transistor to supply the increased load and re-establish the DC output voltage. As the load decreases, the DC output voltage increases which causes the regulator to leave the power transistor on for a shorter period of time to store less energy in the magnetic field. The reduced energy transfer to the secondary winding during the off time of the power transistor causes the power supply to adjust to the decreased load and reduces the DC output voltage back to its steady-state value.
The feedback circuit is configured as either primary side regulation or secondary side regulation. In secondary side regulation, the DC output voltage is monitored directly to develop the feedback signal. In primary side regulation, an auxiliary winding of the transformer develops a feedback voltage proportional to the DC output voltage across the secondary winding as determined by the transformer windings turns ratio. A feedback signal generated by primary side regulation can be of opposite phase with respect to a feedback signal generated by secondary side regulation. That is, a primary side feedback signal may be asserted as a high voltage whereas a secondary side feedback signal may be asserted as a low voltage.
Some prior art regulator circuits are configured as a gated oscillator where the power transistor receives a fixed frequency, fixed duty cycle control signal which is enabled or disabled in response to the feedback signal. One type of gated oscillator regulator circuit accepts primary side regulation while another type of gated oscillator regulator circuit accepts secondary side regulation. Integrated switching regulator circuits typically do not accept both phases of feedback information. To accommodate different types of regulation, external circuitry is used to change the phase of the feedback signal to match the regulator requirements. For example, the feedback signal may be routed through a current mirror to invert the phase. The additional external components add time, cost, and complexity for the power supply manufacturers.
Hence, a need exists for an integrated regulator circuit which accepts either primary side regulation or secondary side regulation directly without using external circuitry to change the phase of the feedback signal.